Mounting a heat sink in thermal contact with an electronic component

ABSTRACT

A heat transfer apparatus comprises a load frame having load springs and an open region that exposes an electronic component. The load frame is mounted to a printed circuit board on which the electronic component is mounted. A heat sink assembly is disposed on the load frame and has a main body in thermal contact with the electronic component through a thermally conductive material. The heat sink assembly has load arms for engaging the load springs. A load plate extends between the load arms and has an actuation element operative to displace the main body relative to the load plate and thereby resiliently deform the load springs and produce a load force that compresses the thermally conductive material to achieve a desired thermal interface gap between the main body and the electronic component. Non-influencing fasteners secure the heat sink to the load frame and maintain the desired thermal interface gap.

BACKGROUND OF THE INVENTION

1 . Field of Invention

The present invention relates in general to the field of electronicpackaging, and in particular to electronic packaging that removes heatfrom an electronic component.

2 . Background Art

Electronic components, such a microprocessors and integrated circuits,must operate within certain specified temperature ranges to performefficiently. Excessive heat degrades electronic component performance,reliability, life expectancy, and can even cause failure. Heat sinks arewidely used for controlling excessive heat. Typically, heat sinks areformed with fins, pins or other similar structures to increase thesurface area of the heat sink and thereby enhance heat dissipation asair passes over the heat sink. In addition, it is not uncommon for heatsinks to contain high performance structures, such as vapor chambersand/or heat pipes, to further enhance heat transfer. Heat sinks aretypically formed of metals, such as copper or aluminum. More recently,graphite-based materials have been used for heat sinks because suchmaterials offer several advantages, such as improved thermalconductivity and reduced weight.

Electronic components are generally packaged using electronic packages(i.e., modules) that include a module substrate to which the electroniccomponent is electronically connected. In some cases, the moduleincludes a cap (i.e., a capped module) which seals the electroniccomponent within the module. In other cases, the module does not includea cap (i.e., a bare die module).

Bare die modules are generally preferred over capped modules from athermal performance perspective. In the case of a capped module, a heatsink is typically attached with a thermal interface between a bottomsurface of the heat sink and a top surface of the cap, and anotherthermal interface between a bottom surface of the cap and a top surfaceof the electronic component. In the case of a bare die module, a heatsink is typically attached with a thermal interface between a bottomsurface of the heat sink and a top surface of the electronic component.Bare die modules typically exhibit better thermal performance thancapped modules because bare die modules eliminate two sources of thermalresistance present in capped modules, i.e., the thermal resistance ofthe cap and the thermal resistance of the thermal interface between thecap and the electronic component. Accordingly, bare die modules aretypically used to package electronic components that require high totalpower dissipation.

Heat sinks are attached to modules using a variety of attachmentmechanisms, such as clamps, screws, and other hardware. The attachmentmechanism typically applies a force that maintains a thermal interfacegap, i.e., the thickness of the thermal interface extending between theheat sink and the module. In the case of a capped module, the capprotects the electronic component from physical damage from the appliedforce. In the case of a bare die module, however, the applied force istransferred directly through the electronic component itself.Consequently, when bare die modules are used, the attachment mechanismtypically applies a compliant force to decrease stresses on theelectronic component.

FIG. 1 illustrates an example of a prior art attachment mechanism forattaching a heat sink to a bare die module using a compliant force. Acircuit board assembly 100 includes a printed circuit board 105 and abare die module 110. Bare die module 110 includes a module substrate115, an electronic component such as a semiconductor chip 120, and anelectronic connection 125. Semiconductor chip 120 is electricallyconnected to module substrate 115. Electronic connection 125, whichelectrically connects printed circuit board 105 to module substrate 115,may be a pin grid array (PGA), a ceramic column grid array (CCGA), aland grid array (LGA), or the like. Semiconductor chip 120 is thermallyconnected with a heat sink 130 through a thermal interface 135, which isa layer of thermally conductive material such as thermal paste, oil, orother high thermal conductivity material. Typically, the thermalinterface 135 is relatively thin so that it may easily transfer heataway from bare die module 110 and toward heat sink 130. The thickness ofthermal interface 135 extending between heat sink 130 and semiconductorchip 120 is referred to as the thermal interface gap.

Heat sink 130 is attached to bare die module 110 using bolts 140. Bolts140 pass through thru-holes 131 in heat sink 130 and thru-holes 106 inprinted circuit board 105 and are threaded into threaded-holes 146 in abackside bolster 145. Typically, bolts 140 are arranged one at eachcorner of the electronic component 120, or one on each side of theelectronic component 120. Bolts 140 are tightened by threading athreaded portion of bolts 140 into threaded-holes 146 in backsidebolster 145. As bolts 140 are tightened, heat sink 130 engagessemiconductor chip 120 through thermal interface 135. Additionaltightening of bolts 140 causes deflection of printed circuit board 105which applies a compliant force to bare die module 110. Moreparticularly, printed circuit board 105 is slightly flexed in aconcave-arc fashion with respect to bare die module 110.

Unfortunately, deflection of printed circuit board 105 will not alwaysprovide the necessary compliance to decrease stresses on electroniccomponent 120. Problems may arise with this board deflection approachif, for example, printed circuit board 105 has a relatively thickcross-section and/or bare die module 110 has a relatively large area(i.e., the “footprint” occupied by bare die module 110 on printedcircuit board 105). Printed circuit boards of relatively thickcross-section are typically less compliant than printed circuit boardsof thinner cross-section. Consequently, the necessary compliance oftencannot be achieved by deflecting relatively thick cross-section printedcircuit boards. Moreover, the resulting stresses on the printed circuitboard upon deflection can lead to catastrophic failure of solder jointsand conductor traces on the printed circuit board. In addition, if thebare die module has a relatively large area, the concave-arc thatresults upon deflection of the printed circuit board can put the solderjoints of the bare die module under unacceptable tension stresses.

SUMMARY OF THE INVENTION

The present invention provides an enhanced mounting mechanism forholding a heat sink in thermal contact with an electronic component in amanner substantially without negative effect and that overcomes many ofthe disadvantages of prior art arrangements.

In accordance with one aspect of the present invention, provision ismade for a heat transfer apparatus comprising: a printed circuit boardhaving an electronic component thereon; a load frame mounted to theprinted circuit board, the load frame having an open region into whichthe electronic component extends exposing a surface of the electroniccomponent, and the load frame having a plurality of load springs mountedthereon; a heat sink assembly disposed on the load frame and having amain body with a surface in thermal contact with the surface of theelectronic component through a thermally conductive material, the heatsink assembly having a plurality of load arms, each load arm having ahook for engaging one of the load springs; and, a load plate extendingbetween the load arms, the load plate having an actuation elementoperative to displace the main body relative to the load plate andthereby resiliently deform the load springs and produce a load forcethat compresses the thermally conductive material to achieve a desiredthermal interface gap between the surface of the main body and thesurface of the electronic component.

In accordance with another aspect of the present invention, provision ismade for a method of assembling a heat transfer apparatus, comprisingthe steps of: attaching a load frame to a printed circuit board havingan electronic component thereon, the load frame having an open regioninto which the electronic component extends exposing a surface of theelectronic component, and the load frame having a plurality of loadsprings mounted thereon respectively configured to receive a pluralityof hooks of a plurality of load arms of a heat sink assembly; dispensinga thermally conductive material on the surface of the electroniccomponent; placing the heat sink assembly on the load frame so that thehooks of the load arms of the heat sink assembly engage the load springsof the load frame and so that a surface of the main body of the heatsink assembly is in thermal contact with the surface of the electroniccomponent through the thermally conductive material; and, actuating anactuation element of a load plate extending between the load arms of theheat sink assembly to displace the main body relative to the load plateand thereby resiliently deform the load springs and produce a load forcethat compresses the thermally conductive material to achieve a desiredthermal interface gap between the surface of the main body and thesurface of the electronic component.

In accordance with yet another aspect of the present invention,provision is made for a heat transfer apparatus comprising: a printedcircuit board having an electronic component thereon; a load framemounted to the printed circuit board, the load frame having an openregion into which the electronic component extends exposing a surface ofthe electronic component; a heat sink disposed on the load frame andhaving a surface in thermal contact with the surface of the electroniccomponent through a thermally conductive material; an actuationmechanism to apply a preload force to the heat sink toward theelectronic component to compress the thermally conductive material andachieve a desired thermal interface gap between the surface of the heatsink and the surface of the electronic component; and, at least onenon-influencing fastener disposed within a bore of the heat sink andthreaded into the load frame to secure the heat sink to the load frameand maintain the desired thermal interface gap.

In accordance with still another aspect of the present invention,provision is made for a method of assembling a heat transfer apparatus,comprising the steps of: attaching a load frame to a printed circuitboard having an electronic component thereon, the load frame having anopen region into which the electronic component extends exposing asurface of the electronic component; dispensing a thermally conductivematerial on the surface of the electronic component; placing a heat sinkon the load frame so that a surface of the heat sink is in thermalcontact with the surface of the electronic component through thethermally conductive material; applying a preload force to the heat sinktoward the electronic component to compress the thermally conductivematerial and achieve a desired thermal interface gap between the surfaceof the heat sink and the surface of the electronic component; and,actuating at least one non-influencing fastener disposed within a boreof the heat sink and threaded into the load frame to secure the heatsink to the load frame and maintain the desired thermal interface gap.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred exemplary embodiments of the present invention willhereinafter be described in conjunction with the appended drawings,where like designations denote like elements.

FIG. 1 illustrates an example of a prior art attachment mechanism forattachment of a heat sink to a bare die module.

FIG. 2 is an exploded perspective view of a heat transfer apparatus inaccordance with the preferred embodiments of the present invention.

FIG. 3 is a perspective view of a heat transfer apparatus shown in FIG.2 with portions of the heat sink removed.

FIG. 4 is a cross-sectional view of a heat transfer apparatus shown inFIG. 2.

FIG. 5 is a cross-sectional view of a portion of the heat transferapparatus shown in FIG. 2 showing a non-influencing fastener arrangementwith a non-influencing fastener in an actuated state.

FIG. 6 is a cross-sectional view of a non-influencing fastener in anon-actuated state.

FIG. 7 is a top plan view of an insulator with an array of sphericaldots according to a preferred embodiment of the present invention.

FIG. 8 is a flow diagram of a method for mounting a heat sink in thermalcontact with an electronic component according to the preferredembodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

1 . Overview

In accordance with the preferred embodiments of the present invention, aheat transfer apparatus comprises a load frame having load springs andan open region that exposes an electronic component. The load frame ismounted to a printed circuit board on which the electronic component ismounted. A heat sink assembly is disposed on the load frame and has amain body in thermal contact with the electronic component through athermally conductive material. The heat sink assembly has load arms forengaging the load springs. A load plate extends between the load armsand has an actuation element operative to displace the main bodyrelative to the load plate and thereby resiliently deform the loadsprings and produce a load force that compresses the thermallyconductive material to achieve a desired thermal interface gap betweenthe main body and the electronic component. Non-influencing fastenerssecure the heat sink to the load frame and maintain the desired thermalinterface gap.

2 . Detailed Description

Reference is now made to FIGS. 2-4 for illustrating a heat transferapparatus 200 in accordance with the preferred embodiments of thepresent invention, which implement an improved process for mounting aheat sink on a heat source, such as an electronic component. FIGS. 2-4are intended to depict the representative major components of heattransfer apparatus 200 at a high level, it being understood thatindividual components may have lesser or greater complexity thatrepresented in FIG. 2-4, and that the number, type and configuration ofsuch components may vary. For example, heat transfer apparatus 200 maycontain a different number, type and configuration of heat sources(e.g., electrical components) than shown.

As best seen in FIG. 2, which is an exploded perspective view of heattransfer apparatus 200, heat transfer apparatus 200 comprises two maincomponents, i.e., a load frame/spring assembly 202 and a heat sink/loadarm assembly 204. Load frame/spring assembly 202 includes a load frame206 and a pair of load springs 208. Load frame 206 is preferably made ofan alloy material chosen for its low creep properties, such Zamak 8.Zamak 8, also known as ZA-8, is the trade name for a zinc-based alloy,the primary components of which are zinc, aluminum, copper, andmagnesium. Creep is the development over time of additional strains in amaterial. Creep depends on the magnitude of the applied force and itsduration, as well as the temperature and pressure. A material havinghigh creep resistance is preferable in the construction of load frame206 because creep deformation is to be avoided.

Load springs 208 are preferably made of an alloy material chosen for itshigh tensile strength properties, such as high strength music wire.Although two load springs 208 are shown in FIG. 2, those skilled in theart will appreciate that the present invention may be practiced with anynumber of load springs 208 (and load arms 210, which engage the loadsprings 208 as described below in the discussion of the heat sink/loadarm assembly 204).

Load frame 206 is mounted on a printed circuit board 212. Referring nowtemporarily to FIG. 4, fasteners such as screws 410 (two of which aredenoted with a dotted line in FIG. 4) are used to attach load frame 206to printed circuit board 212. Screws 410 are denoted with a dotted linein FIG. 4 because they do not lie in the cross-section shown in FIG. 4.Preferably, four screws 410 (i.e., one near each corner of load frame206) pass through thru-holes in a backside stiffener 412, an insulator414 such as a polyimide, and printed circuit board 212, and are receivedin threaded holes in load frame 206. This configuration advantageouslyallows access to screws 410 even when the heat sink/load arm assembly isattached to the load frame/spring assembly.

Now returning to FIG. 2, load frame 206 includes one or more openregions 214 into which extends the heat source, e.g., an electroniccomponent (not shown in FIG. 2) mounted on printed circuit board 212.For example, a bare die module may be mounted on printed circuit board212 at the location designated at the intersection of the cross-hairs isshown in FIG. 2.

As shown in FIG. 2, load frame 206 includes four mounting projections216 to which the ends of load springs 208 are secured. Load frame 206also includes two downstop support projections 218 on which rest themid-sections of load springs 208.

One or more non-influencing fasteners 220 are used to secure heatsink/load arm assembly 204 to load frame/load arm assembly 202.Preferably, four non-influencing fasteners 220 are mounted on load frame206. Each of the non-influencing fasteners 220 is preferably threadedinto a boss 416 (shown in FIG. 4) of load frame 206. The non-influencingfasteners (NIFs) lock the heat sink in position without influencing theposition of the heat sink.

Heat sink/load arm assembly 204 includes a heat sink 224 having a baseplate 226. Preferably, heat sink 224 is formed with fins, pins or othersimilar structures to increase the surface area of the heat sink andthereby enhance heat dissipation as air passes over the heat sink. It isalso preferable for heat sink 224 to contain high performancestructures, such as vapor chambers and/or heat pipes, to further enhanceheat transfer. For example, heat sink 224 may contain one or more vaporchambers (not shown) filled with deionized water. Heat sink 224 may, forexample, be formed of metals, such as copper or aluminum, or othermaterials, such as graphite-based materials.

As mentioned above, heat sink/load arm assembly 204 includes load arms210. Load arms 210 are pivotally attached to a U-channel load plate 228.Load arms 210 and U-channel load plate 228 may be made of stainlesssteel, for example, and are preferably configured to provide minimal airflow impedance across the fins of heat sink 224. For example, load arms210 preferably have an open area through which air may flow. When heatsink/load arm assembly 204 is attached to load frame/spring assembly202, load arms 210 engage the load springs 208. This engagement isdescribed in detail below in the discussion of the actuation mechanismwith reference to FIGS. 3 and 4. In addition, when heat sink/load armassembly 204 is attached to load frame/spring assembly 202,non-influencing fasteners 220 are received in bore holes 230 in the heatsink's base plate 226. This non-influencing fastener arrangement isdescribed in detail below with reference to FIGS. 4-6. To aid inalignment of heat sink/load arm assembly 204 with respect to loadframe/spring assembly 202, load frame 206 may include alignment pins 232that are received in corresponding alignment holes (not shown) in theheat sink's base plate 226.

FIG. 3 is a perspective view of a heat transfer apparatus 200 withportions of heat sink 224 removed. FIG. 4 is a cross-sectional view of aheat transfer apparatus 200. As best seen with reference to FIGS. 3 and4, an actuation mechanism applies a preload force to heat sink 224toward a semiconductor chip 402 (shown in FIG. 4) to compress athermally conductive material 408 (shown in FIG. 4) and achieve adesired thermal interface gap between heat sink 224 and semiconductorchip 402. The main components of the actuation mechanism include loadframe 206, the load frame's mounting projections 216, load springs 208,load arms 210, the load arms' hook portions 310, hinge pins 312,U-channel load plate 228, actuation screw 314, push plate 420, the pushplate's guide pins 234, heat sink 224, and the heat sink's base plate226. Referring to FIG. 3 for the moment, load arms 210 each include ahook portion 310 that engages one of the load springs 208. Load arms 210are pivotally attached to U-channel load plate 228 by hinge pins 312. Anactuation screw 314 is threaded through U-channel load plate 228 toengage an underlying push plate 420 (shown in FIG. 4). Actuation screw314 may be, for example, an M3 screw. Actuation screw 314 is accessiblefor actuation from the top of U-channel load plate 228. The distancebetween the U-channel plate and push plate 420 is adjusted by turningactuation screw 314. This provides a controlled rate of loading. Thoseskilled in the art will recognize that other actuation elements andtechniques to provide a controlled rate of loading are possible withinthe scope of the present invention, such as camming, rocking and thelike.

Still referring to FIG. 3, when the load frame/spring assembly and theheat sink/load arm assembly are brought together, hook portions 310 ofload arms 210 are engaged with load springs 208, and the actuationmechanism is actuated by turning actuation screw 314 in a direction toincrease the distance between U-channel load plate 228 and theunderlying push plate 420 (shown in FIG. 4). Load springs 208 aredeflected by actuation of the actuation mechanism. The geometricparameters of load springs 208, i.e., the span, cross-section profile,and diameter) are optimized for the allowable space within theapplication and the required resulting load. Force is transmittedthrough the heat sink's fins and base plate 226 onto the underlyingsemiconductor chip 402 (shown in FIG. 4). The force compresses athermally conductive material 408 (shown in FIG. 4) and achieves adesired thermal interface gap between heat sink's base plate 226 andsemiconductor chip 402.

Referring now to FIG. 4, push plate 420 is affixed to heat sink 224. Forexample, push plate 420 may be soldered to heat sink 224 using, forexample, SAC 305 solder. Alternatively, push plate 420 may be affixed toheat sink 224 with a suitable adhesive, such as epoxy. Push plate 420may be made of stainless steel, for example. Preferably, push plate 420is affixed in a location directly above the heat source, with the widthof U-channel load plate 228 and push plate 420 substantially capturingthe footprint of the heat source. This provides centroid loading abovethe bare die module, and thus provides substantially no edge stresses onthe die. As shown in FIG. 4, for example, push plate 420 is affixed toseven of the heat sink's fins that lie above semiconductor chip 402.Although not shown in FIG. 4, additional modules residing on printedcircuit board 212 may be accommodated in open area 214 of load frame206. In such a case, push plate 420 is preferably affixed in a locationdirectly over the primary module, with the width of U-channel load plate228 and push plate 420 substantially capturing the footprint of theprimary module.

As best seen in FIGS. 2 and 3, push plate preferably includes guide pins234 that extend through corresponding holes in U-channel load plate 228.The purpose of guide pins 234 is to align push plate 420 relative toU-channel load plate 228.

As shown in FIG. 4, in the preferred embodiments, the heat source is oneor more bare die modules, each bare die module comprising an electroniccomponent such as a semiconductor chip 402, a module substrate 404, andan electronic connector 406. However, those skilled in the art willappreciate that the present invention may be practiced using other typesof heat sources such as one or more capped modules and/or otherelectronic components. The bare die module shown in FIG. 4 is asingle-chip module (SCM); however, those skilled in the art willrecognize that the spirit and scope of the present invention is notlimited to SCMs. For example, those skilled in the art will recognizethat the present invention may be practiced using one or more multi-chipmodules (MCMs), or a combination of MCMs, SCMs and/or other electroniccomponents/heat sources.

It is significant to note that the present invention allows a singleheat transfer apparatus to accommodate one or more modules havingdifferent footprints. Previous solutions required qualification ofindividual modules based on differences in footprint. The presentinvention overcomes this drawback of the prior art.

The bare die module is conventional. Semiconductor chip 402 iselectrically connected to module substrate 404. Electronic connector406, which electrically connects printed circuit board 212 to modulesubstrate 404, may be a pin grid array (PGA), a ceramic column gridarray (CCGA), a land grid array (LGA), or the like.

In some cases, electronic connector 406 may be susceptible to beingcrushed by the force applied by the actuation mechanism. This isproblematic not only from the perspective of possible damage toelectronic connector 406, but also throws off the planarity of the stack(i.e., the module substrate 404 and semiconductor chip 402) relative tothe heat sink's base plate which causes thermally conductive material408 to form an uneven thermal interface gap. In such cases, one or morecrush protection elements 422 (denoted with a dotted line in FIG. 4 dueto the optional nature thereof) may be inserted along peripheralportions of module substrate 404 between the bottom of module substrate404 and the top of printed circuit board 212. The crush protectionelements 422 may be made of a material such as a polythermal plastic orthe like.

As shown in FIG. 7, additional crush protection may be utilized in theform of spherical dots 710 applied to insulator 414. Spherical dots 710are made of a compliant material such as epoxy or the like, and aredeposited on insulator 414 in an area that underlies the bare diemodule. For example, spherical dots 710 may be arranged in an arraypattern adjacent to the edge of open region 214 of the load frame. Thearray pattern shown in FIG. 7 is exemplary, one skilled in the art willappreciate that numerous other array patterns are possible. Thecompliant nature of spherical dots 710 functions to improve planarity ofthe thermal interface gap formed by the thermally conductive material.

Referring back to FIG. 4, thermal interface 408 is made of a thermallyconductive material such as thermal gel, grease, paste, oil, or otherhigh thermal conductivity material. Preferably, thermal interface 408 ismade of Shin-Etsu gel or grease with aluminum and/or zinc oxide spheres.Typically, thermal interface 408 is relatively thin so that it mayeasily transfer heat away from semiconductor chip 402 and toward theheat sink's base plate 226. The thickness of thermal interface 408extending between the bottom of the heat sink's base plate 226 and thetop surface of semiconductor chip 402 is referred to as the thermalinterface gap. Preferably, the thermal interface gap is about 1.2 mil.

Thermally conductive material 408 is dispensed on semiconductor chip 402prior to bringing the load frame/spring assembly and the heat sink/loadarm assembly together. To protect semiconductor 402 as these assembliesare initially brought together, a viscoelastic foam pad 430 may beinterposed between the lower surface of the heat sink's base plate 226and the upper surface of load frame 206.

Those skilled in the art will appreciate that the actuation mechanismshown in FIGS. 3 and 4 is exemplary, and that other actuation mechanismsmay be used to apply the preload force within the spirit and scope ofthe present invention. According to one embodiment of the presentinvention, once the preload force is applied to achieve the desiredthermal gap, irrespective of the actuation mechanism that applied thepreload force, one or more non-influencing fasteners are actuated tosecure the heat sink to the load frame and maintain the desired thermalgap.

As shown in FIG. 4, when the heat sink/load arm assembly is attached tothe load frame/spring assembly, non-influencing fasteners 220 arereceived in bore holes 230 in the heat sink's base plate 226. Once theactuation mechanism applies the preload force to achieve the desiredthermal interface gap, non-influencing fasteners 220 are actuated tosecure heat sink 224 to load frame 206 and maintain the desired thermalgap. The non-influencing fastener arrangement is shown in more detail inFIGS. 5 and 6. FIG. 5 shows the non-influencing fastener arrangementwith non-influencing fastener 220 in an actuated state. FIG. 6 showsnon-influencing fastener 220 in a non-actuated state. Non-influencingfastener 220 includes a screw 510 that is threaded into one of thebosses 416 of load frame 206. Captivated on screw 510 are a split taperring 520 and a solid taper ring 530. Preferably, the taper of splittaper ring 520 matches that of solid taper ring 530. Non-influencingfastener 220 is accessible through bore hole 230 in the heat sink's baseplate 226, and is actuated by turning screw 510 into the load frame'sboss 416 so that split taper ring 520 is expanded against the wall ofbore hole 230 in the heat sink's base plate 226. Non-influencingfasteners 220 are advantageous because they can be actuated withoutsignificantly altering the thermal interface gap, as would be the casewith a conventional fastener.

FIG. 8 is a flow diagram of a method 800 for mounting a heat sink inthermal contact with an electronic component according to the preferredembodiments of the present invention. Method 800 sets forth thepreferred order of the steps. It must be understood, however, that thevarious steps may occur at any time relative to one another. The baredie module is soldered to the printed circuit board (step 810). If acrush protection element is desired, the crush protection element isinserted along peripheral portions of the module substrate between thebottom of module substrate and the top of printed circuit board (step820). The load frame is attached to the printed circuit board (step830). Thermally conductive material is dispensed on the semiconductorchip (step 840). Next, the heat sink/load arm assembly is aligned andbrought into contact with the load frame/spring assembly (step 850).During step 850, the hook portion of each load arms is brought intoengagement with one of the load springs. Method 800 continues with theapplication of a preload force using the actuation mechanism to set thethermal interface gap (step 860). During step 860, the actuation screwis turned an appropriate amount to apply a preload force (e.g., 40 lbs)that provides the desired thermal interface gap (e.g., 1.2 mil). Inother words, some of the thermally conductive material is squeezed-outby the preload force to provide the desired thermal gap. Once this pointis reached, the assembly may optionally be thermally cured to set thethermal interface gap. Next, the non-influencing fasteners are actuatedto secure the heat sink to the load frame and maintain the desiredthermal gap (step 870). Preferably, an appropriate torque is applied tothe non-influencing fasteners using an X-pattern sequence to minimizethe application of any stresses.

Thermal sensors may be used to measure the thermal interface gapachieved by method 800. If the desired thermal interface gap is notachieved, the unit may be simply reworked by removing the heat sink/loadarm assembly from the load frame/spring assembly, and cleaning thethermally conductive material from the semiconductor chip, and returningto step 840.

One skilled in the art will appreciate that many variations are possiblewithin the scope of the present invention. For example, othernon-influencing fastener arrangements may be used in lieu of thenon-influencing fastener arrangement described above. Moreover, althoughnon-influencing fasteners are preferable, adhesive such as a pressuresensitive adhesive, UV-sensitive adhesive, thermal curing adhesive,epoxy, or any other suitable adhesive may be used in lieu of thenon-influencing fasteners described above. Thus, while the presentinvention has been particularly shown and described with reference topreferred embodiments thereof, it will be understood by those skilled inthe art that these and other changes in form and details may be madetherein without departing from the spirit and scope of the presentinvention.

1. A heat transfer apparatus comprising: a printed circuit board havingan electronic component thereon; a load frame mounted to the printedcircuit board, the load frame having an open region into which theelectronic component extends exposing a surface of the electroniccomponent; a heat sink disposed on the load frame and having a surfacein thermal contact with the surface of the electronic component througha thermally conductive material; an actuation mechanism to apply apreload force to the heat sink toward the electronic component tocompress the thermally conductive material and achieve a desired thermalinterface gap between the surface of the heat sink and the surface ofthe electronic component; and at least one non-influencing fastenerdisposed within a bore of the heat sink and threaded into the load frameto secure the heat sink to the load frame and maintain the desiredthermal interface gap, wherein the at least one non-influencing fastenerincludes the combination of a screw, a split taper ring, and a solidtaper ring, and wherein compression of the rings by turning of the screwinto the load frame caused the split taper ring to expand against thebore of the heat sink.
 2. A method of assembling a heat transferapparatus, comprising the steps of: attaching a load frame to a printedcircuit board having an electronic component thereon, the load framehaving an open region into which the electronic component extendsexposing a surface of the electronic component; dispensing a thermallyconductive material on the surface of the electronic component; placinga heat sink on the load frame so that a surface of the heat sink is inthermal contact with the surface of the electronic component through thethermally conductive material; applying a preload force to the heat sinktoward the electronic component to compress the thermally conductivematerial and achieve a desired thermal interface gap between the surfaceof the heat sink and the surface of the electronic component; andactuating at least one non-influencing fastener disposed within a boreof the heat sink and threaded into the load frame to secure the heatsink to the load frame and maintain the desired thermal interface gap,wherein the step of applying a preload force includes the step ofturning an actuation screw, and wherein the step of actuating at leastone non-influencing fastener includes the step of turning a plurality ofNIF screws each expanding a split taper ring against the bore of theheat sink, and wherein the actuation screw and the NIF screws areaccessible and turned from the side of the printed circuit board onwhich the electronic component resides.